The present invention relates to optical systems used in an electrophotographic reproduction device for exposing an original document on a document platen and, more particularly, to an apparatus for sensing the amount of contamination present in the optical system.
In an electrostatographic reproducing apparatus of the type commonly in use today, a photoconductive insulating member is charged to a uniform potential and thereafter exposed to a light image of an original document to be reproduced. The exposure selectively discharges the photoconductive insulating surface and creates an electrostatic latent image on the member which corresponds to the image areas contained within the original document. Subsequently, the electrostatic latent image on the photoconductive insulating surface is made visible by developing the image with a developing powder, referred to in the art as toner. Most development systems employ a developer material which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charge pattern of the image areas on the photoconductive insulating surface to form a powder image. This image may subsequently be transferred to a support surface such as copy paper to which it may be permanently affixed by heating or by the application of pressure. Following transfer of the toner image to the support surface, the photoconductive insulating surface is cleaned of residual toner to prepare it for the next imaging cycle.
One of the problems associated with these prior art reproduction machines is contamination of the various processing stations by charged toner particles, paper particles, dust particles and the like. These particles may be attracted to critical surfaces of the various processing stations, resulting in contamination and degradation of the performance of that subsystem. To maintain copy quality, it is essential that the elements of the automatic reproducing machine remain substantially free from contaminating particles. One of the areas which is most sensitive to contamination is the optical system. If toner or dust is allowed to collect on a lens or a mirror, for example, the exposure is reduced by light that is scattered out of the optical path causing copy background and, further, image modulation is reduced by the same phenomenon of light being scattered into the image foreground areas. Either factor results in a loss of low contrast copy ability.
It is known in the art to use a photosensor to sense the presence of a buildup in optical system contamination. The sensors are typically used to measure a change in one of the following parameters: reflectance of a mirror; transmittance of a transparent material on the light scattering surface, or transmission of a lens. A feedback control system is usually implemented to increase the illumination source output to correct the exposure. U.S. Pat. No. 4,555,621, is illustrative of such prior art sensing systems.
These prior art sensing and control systems have several disadvantages. Both photosensor and illumination light sources are subject to long term sensitivity/emission variations. When a correction occurs, the signal change cannot be differentiated from a signal change attributable to contamination sensing. Thus, when a correction signal is generated by the sensing/control system, it is not always certain that the sensed correction condition is actually due to increased contamination in the system or to the variation in the sensing/control system. Thus, an unnecessary cleaning procedure may be initiated which, furthermore, sometimes results in inadvertent damage to other system components. Another disadvantage is that the illumination exposure control circuit, while restoring the correct exposure does not improve the contrast degradation caused by the contamination. In fact, contrast degradation, as will be described below, can only be corrected by cleaning the optical elements and the need for improvement is a determination of an optimum time doing so.
The present invention is, therefore, directed to a contamination sensing system which is not susceptible to drift variations in the sensing components, to lamp drift or other conditions not directly related to the actual contamination present in the system. The present sensing system is designed to generate a correction signal only when contamination reduces image modulation as expressed by an MTF value below a previously set minimum value. This sensing system includes a low resolution sensor array such as a CCD array to measure image modulation at some point along the optical path. This is done by sensing a bar target of known resolution which is placed on the document platen. More particularly, this invention relates to an electrophotographic reproduction machine including imaging means for forming a latent image of a document located in an object plane, at a photoreceptor surface, the imaging means including means for illuminating the document and means for projecting the document image onto said photoreceptor surface, the improvement comprising the combination of a linear sensor array positioned along the optical path, said sensor array adapted to read a bar chart located in the object plane and to generate output signals representative of said bar chart maximum and minimum values, a comparison/memory circuit which receives the output signals from said sensor array and computes the value of the modulation transfer function MTF, said circuit containing nonvolatile memory with a predetermined value representing a minimum MTF value stored therein, said memory adapted to compare the previously stored MTF value with the computed value, and output means connected to said comparison/memory circuit, said output means actuated to provide an indication of when the measured MTF falls below the predetermined minimum MTF value.